Gasification of low calorific value solid fuels to produce electric energy

ABSTRACT

A process for producing fuel gases from a solid fuel including carbonaceous material and having a calorific value of between about 800 to 3,000 Kcal/Kg. The process includes a) pyrolyzing the solid fuel under reaction conditions and for a time sufficient to pyrolyze about 40-80% of the carbonaceous material whereby to form a first gas and a carbon residue without formation of tar, and b) gasifying carbon residue to form a second gas by heating the carbon residue in the presence of oxygen or oxygen-steam; or burning the carbon residue. In a preferred embodiment, the first and second gases are mixed under conditions to neutralize contained sulfur gases by a Claus reaction.

This application is a 371 of PCT/GR94/00011 filed Jun. 3, 1994.

This application is a 371 of PCT/GR94/00011 filed Jun. 3, 1994.

FIELD OF INVENTION

The invention describes an original gasification method of low calorificvalue solid fuels e.g. lignites and peats with pyrolysis and oxygen oroxygen-steam gasification in two stages. Additionally it describes anoriginal process by which the gases produced are utilised a Co-Gasadvanced system for producing high amounts of electric energy in anoperation running without environmental pollution.

BACKGROUND OF INVENTION

With the existing crisis in securing adequate amounts of energy andsince petroleum supplies are not regular in availability and price, thenational programs for producing electric energy rather prefer to developlocal energy sources. In this preferred development, coal is the mainsource to consider which is the first fuel to be used in powerproduction and is more abundant and more regularly distributed in theWorld than oil. The resources in coal are divided in low and highthermal value. They are also divided according to their sulfur contentwhich by burning the solid fuels becomes sulfur dioxide creating toxicenvironmental pollution. With that problem, the utilization of solidfuels is restricted to those containing low sulfur and create as low aspossible environmental damage.

In relation to coal and to its utilization in the production of electricenergy it is observed that by its burning, the result in electric energyis low, it releases high amounts of sulfur dioxide, fly ash and nitricoxides and it creates high corrosion in the equipment.

Additionally, by burning solid fuels high amounts of carbon dioxide areproduced which today are considered a major pollution factor, being themain source for the green-house conditions emerging in our Planet. Andall these environmental and production problems appear more critical bythe use of solid fuels of low calorific values such as lignites andpeats.

To face these problems today there exist solutions leading to thereduction of the sulfur content in those low calorific fuels and to theneutralization of the combustion gases.

Those solutions, however, are costly and the corrections offered,because of cost, do not make them attractive. A better approach appearsto be the gasification of those low calorific fuels as an actionattractive today in spite of its leading to high losses of energy. Withtotal gasification the gases can be washed to separate them from thetoxic gases and the flying ash but with total gasification the thermalvalue is further reduced to 65-70% and expensive industrialinstallations are needed in the operation.

In the meantime, however, with the development of gas turbines in theproduction of power more economical solutions are available to utilizegases. Our original solution is such a method which utilizes the fuelgases produced in Co-Gas advanced systems by which the degree ofproduced electric energy with the use of air turbines and combined cycleis improved. For operating the gas turbines, however, we need fuel gasesfree of corrosives and free of tars and liquid byproducts, but also ofthe highest possible thermal value.

SUMMARY OF INVENTION

Considering those developments, the technological characteristics oflignites and peats of low calorific value have been studied and it hasbeen discovered that those solid fuels either as they are received orafter deashing (described in another invention) show high efficiency inrunning in such an advanced system for producing electric energy becausethose fuels are pyrolyzed at high extent (40-85%), highly exothermallywithout forming tars and liquid byproducts. The pyrolysis of those lowcalorific value fuels is optimized at 400°-600° C., and the pyrolytictreatment is highly exothermic in character. The pyrolysis residue isreceived in high carbon purity with thermal value of 4.000-6.000 Kcal/Kgwithout ash, or with 2.200-4000 Kcal/Kg with ash. It has been studiedfor the described invention the gasification of that carbon residue withoxygen and preferably with oxygen-steam and has been discovered that thefuel gases produced are of extremely high thermal value and received athigh temperatures of 900°-1.000° C. and that the gasification achievesthe complete utilization of carbon. According to this procedure it hasbeen discovered for the said invention that the two-stage gasificationof lignites and peats achieves a very high thermal efficiency, and theoxidative gasification does not lead to tars or liquid byproducts.

It has been discovered for the present invention that the pyrolytictreatment proceeds exothermally producing 350-600 Kcal/Kg at 600° C. andthe exothermic output in energy is related to the degree of pyrolysis.To that quantity of energy is added the thermal content of the fuelgases and the thermal exchange of the bottom ash and the fuel gasesproduced in the oxidative gasification.

More heating needs can be adopted on the incoming solid fuels as shownin diagram 1. Thus, the conditions by which the thermal balance of thepyrolytic treatment is covered without using carbon thermal energy havebeen also studied. And this leads to high energy economy and to highenergy utilization of the low calorific solid fuels.

The two stages of gasification, the pyrolysis treatment and thegasification proper (with oxygen or with oxygen-steam) have beendiscovered in the present invention beneficially to lead to products ofdifferent chemical character. Pyrolysis is a reductive treatment inwhich sulfur is gasified as hydrogen sulfide and the oxidative treatmentis oxidative in chemical character in which sulfur is gasified as sulfurdioxide. The inventor has discovered a solution for neutralizing therespective sulfur gases by creating conditions to run the fuel gases toa Claus reactor. With mixing those fuel streams, after first utilizationof their thermomechanical energy in a turbine system, with temperaturesof 600° C. and at a pressure of 30 at, and feeding them to a Clauscatalytic reactor the sulfur gases release product sulfur

    2H.sub.2 S+SO.sub.2 →3S+2OH.sub.2

That possibility to neutralize the sulfur gases beneficially simply withproduction of valuable sulfur is a main advantage of the invention. Itsatisfies a goal for R & D activity: to develop a method of producingelectric energy from low calorific value solid fuels which does notcreate toxic pollution problems from sulfur dioxide and from flying ash.The derived sulfur is collected in high purity and may in small amountsbe taken with the gases flow from where it is washed with water andcollected.

Another advantage of the invention is that the fuel gases are receivedto pressures of 30 at. developed during the pyrolytic treatment and forma working pressure in the two gasification treatments and in the Clausunit. The fuel gases are received at temperatures of 600° to 900° C. andat pressure 30 at., free of corrosive substances and sulfur gases.

Another basic advantage of the invention is the experimentally provedevidence that the low calorific value solid fuels (lignites and peats)are pyrolyzed exothermally because of the oxygen content of the organicmaterials, which resemble wood.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a diagrammatic representation of the gasification andutilization of lignites and peats according to a preferred embodiment ofthe claimed method.

DETAILED DESCRIPTION

Wood and woody biomass are known to pyrolyze exothermally attemperatures higher than 400° C. and that has been utilized beneficiallyin the past for the distillation treatment of wood and recently in thepyrolytic treatment of garbage biomass. The low calorific value solidfuels (lignites and peats) have the following woody consistency.

                  TABLE 1                                                         ______________________________________                                        The consistency of Lignites and Peats                                         Constituents      Lignites Peats                                              ______________________________________                                        pH                5.8-6.9  4.6-5.4                                            Ash               15-35      6-20.5                                           Waxy substances etc.                                                                            5.2-6.8  8.1-8.3                                            Humic acids         20-33.8                                                                                18-34.1                                          Humins            30-40      37-42.1                                          Holocellulose     31-35    26.1-32.9                                          d-cellulose        8-15    10.5-12.0                                          ______________________________________                                    

With the above which determine the nature of the pyrolytic tendency andthe result of the gasification with oxygen or with oxygen and steam asystem is formed with profitable thermal balance in thermal exchangesand final results. The thermal operational parameters determine:

a. That the heating of the solid fuels to the pyrolytic treatment isaffected by the rejected thermal energy, that is thermal energy fromoff-gases, bottom ash, etc.

b. That the pyrolytic gasification is exothermic, producing 250-600Kcal/Kg thermal energy with formation of operational pressures up to 30atm. and it is advanced without being influenced by moisture or ashpresence and it is a reaction of reductive chemical character.

c. That the Claus reaction of neutralizing the sulfur gases isspontaneous at temperatures 600° C. and at pressures of 30 at. of thefuel gases and provided that the molar ratio of H2S/SO2 is 2:1, thereaction is quantitive.

d. That the installation for utilizing the procedure should operateunder pressure 30 at. and at temperature of fuel gases up to 900° C.

The drying of the solid fuels e.g. lignites or peats as they are orafter a deashing treatment with the fuels pulverized form, first withmechanical dewatering and then with heating to 180°-300° C. withexchange of the ash thermal energy received at 1.000° C. and of thethermal energy of the off-gases so that to be finally received as offgases at 180°-300° C.

The pyrolytic treatment starts with the solid fuel e.g lignite attemperature 180°-300°, while to be pyrolyzed, temperatures of 450° to600° are needed. To form those temperatures the following thermalsources are used a) that of exchange on the gases of the oxidativegasification which are received at 1000° C. and can offer 200° C. to thepyrolysis mass (cooled down to 600° C.) and b) that of the thermalenergy resulting from the exothermal pyrolytic reaction which willincrease temperatures by 200° to 300° C.

With those thermal offers the pyrolytic treatment attains temperaturesof 600° C. and higher. The energy coverage of the pyrolytic treatment iscontrolled by heating arrangements on the incoming lignite if needed,nevertheless, this is depending largely on the relative extent of thepyrolysis and of the oxidative gasification treatments.

The gasification of the carbon pyrolysis residue with oxygen orpreferably with oxygen steam is added at 600° C. with high carbon purityand in porous stage proceeds very energetically with quantitativetransformation of the contained carbon and rapid increase of thetemperature to 900°-1000° C. The losses in thermal energy at theoxidative treatment are comparably low, lower than 12% and this refersto the 50% of total. The actual thermal energy loss is under 6% which islow for total gasification treatment and a high energy benefit.

The two streams of gasses the one from pyrolysis and the one fromgasification with oxygen or with oxygen steam are mixed as they arereceived or after energy exchange utilization in a turbine. They arethen directed to the Claus unit which operates under pressure. In theClaus unit the sulfur gases are neutralized and the fuel stream isreceived free of corrosive gases.

An analysis of gases produced in the two reactors that of pyrolysis andthat of oxygen gasification for a number of greek lignites and peats aregiven in the following Table 2 as maxima and as minima of composition.

                  TABLE 2                                                         ______________________________________                                        The composition of the gas fuels from pyrolysis and                           oxygen gasification                                                                                    From Oxygen                                          From Pyrolysis, %        Gasification, %                                      ______________________________________                                        Methane      30-35%      Carbon monoxide                                      Carbon monoxide                                                                            30-50%      35-40%                                               Carbon dioxide                                                                             2-6%        Carbon dioxide                                       Hydrogen     16-22%      16-22%                                               Hydrolgen Sulfide                                                                          1-3%        Hydrogen 40-60%                                                               Sulfur dioxide 1-2%                                  ______________________________________                                    

The procedure of the pyrolytic reaction on a number of solid fuels oflow thermal value gave the results of Table

                  TABLE 3                                                         ______________________________________                                        The pyrolytic reaction of low caloric value lignites                          and peats in % (free of ash and in dry form)                                                    Ptolemais                                                                              Megalopolis                                                                             Aliveri                                                    (North   (Peloponessus,                                                                          (Euboea,                                 temperature                                                                             Peat    Greece)  Greece)   Greece)                                  ______________________________________                                        400°                                                                             15.2%   17.3%    35.4%     16.8%                                    450°                                                                             22.4    23.5     44.3      23.4                                     500°                                                                             34.24   35.28    52.4      37.2                                     550°                                                                             34.48   39.43    67.42     44.64                                    600°                                                                             44.00   44.24    75.42     51.00                                    650°                                                                             44.63   46.6     79.38     56.00                                    Ash content                                                                             11.55%  10.8%    20.6%     11.5%                                    Kcal/Kg of                                                                              4.400   5.100    4.400     5.400                                    the solid fuel                                                                Kcal/Kg of the                                                                          4.465   5.200    4.020     5.730                                    coal residue                                                                  ______________________________________                                    

In the drawing, the utilization of the gases produced for electricityproduction are easily recognized as are the energy benefits obtainedaccording to the present invention.

The production sequence consists of two pressure reactors in series thatof pyrolysis and that of gasification with oxygen. The pyrolysis reactoris designed to operate at a temperature of 700° and pressure of 50 atmand is of fluidized bed type with automated systems for carbon feeding,and for withdrawing the products obtained: the carbon residue and thefuel gases.

The gasification reactor is designed to operate at temperatures up to1200° and at pressures up to 50 atm and it is of solid bed type withautomated systems for feeding and introducing oxygen and for releasingash and the gases produced.

Another possibility for applying the present invention is a combinationof the pyrolytic treatment with burning the carboneous residue in theexisting boiler producing pressure steam.

According to this solution the solid fuels e.g. lignites or peats areintroduced to the pyrolysis reactor with moisture up to 60% or in dry orsemidried form and the fuel gases produced are fed to a turbine forutilization of their thermomechanical energy then are washed and thehydrogen sulfide present is neutralized by known procedures such as in acombination with the Stratford process. The fuel gases after this areburned to produce high amounts of electric energy in a combined-circleadvanced system. The carbonareous residue in this case is burned in theexisting boiler to produce pressure steam to run existing steam turbineor newly installed. With that solution the electric energy output isabout three times higher than the one obtained today and thedesulfurization covers the 70% of sulfur total presence in the solidfuel.

In the present invention it has been shown that the pyrolytic treatmentis not influenced by the moisture of ash presence and that thistreatment makes an energetic transformation pattern because the energyuse is taken by the products produced, the gases and the carbon residue,and the steam formed actually increases substantially the gas volume andtheir energy content. Apart from utilizing the solid fuel optimized bybiorefining release, the exothermic reaction is a substantialcontribution in energy quantity and as energy source.

The fuel gases from the reactors are mixed and directed to a turbine torelease part of the thermomechanical energy as electrical energy andthen are introduced to a Claus reaction unit. In the Claus unit thegases for optimization should have a temperature of 400-450 and aworking pressure. The thermomechanical energy can be also used in steamgeneration by thermal exchange.

At the end the fuel gases contain thermal energy up to 95%+ of thermalenergy of the initial solid fuel in biorefining utilization and inexothermic reaction energy addition.

The fuel gases are fed into an advanced combined circle utilization forelectric energy output. This, according to this invention, can exceedthe 65% in combination of the turbine for thermomechanical energyutilization.

The yield in electrical energy today is 1.1 Kg of 3.000 Kcal lignite perKWh or with lignites and peats of thermal content 800-1200 Kcal/Kg theyield is 1.8-4.1 Kg/KW of electric energy. With the described inventionthe yield in electricity is impressively high, 0.41-0.62 Kg of ligniteor peat/KWh since the lignites and the peats of low calorific contentare utilized according to their energy content in dry form andadditionally by the contribution of a sizable exothermic reaction whichadds 20-30% in energy increase. In view of the above, it can beappreciated that the present invention, in utilizing low caloric solidfuels with pyrolytic tendency of 30% to 80%, advances high yields inelectricity production which is comparable to solid fuels of highthermal value and to oil, in an operation beneficially running entirelypollution free.

The present invention, therefore, introduces a procedure for electricityproduction of low cost from low calorific solid fuels which have a widedistribution in all the World in an operation which although it produceshigh amounts of electricity also introduces an operation running free ofpollution from flying ash and from SO2 and can be arranged also to befree of nitric oxides, thus to be entirely pollution free. It also leadsto a visable reduction of CO2 release of 75% per production unit.

After the description of the invention and the examples and the drawingswhich determine it, it is claimed:
 1. A process for producing fuel gasesfrom a solid fuel comprising carbonaceous material and having acalorific value of between about 800 to 3,000 Kcal/Kg, the processcomprising:a) pyrolyzing the solid fuel under reaction conditions andfor a time sufficient to pyrolyze about 40-80% of the carbonaceousmaterial whereby to form a first gas and a carbon residue withoutformation of tar; and b) gasifying the carbon residue by heating thecarbon residue in the presence of oxygen or oxygen-steam; or burning thecarbon residue.
 2. A process for producing fuel gases as claimed inclaim 1 wherein the solid fuel is lignite or peat.
 3. A process forproducing fuel gases as claimed in claim 2 wherein the solid fuel ispyrolyzed in a reductive reaction to form hydrogen sulfide in the firstgas and the carbon residue is gasified in an oxidative reaction to forma second gas containing sulfur dioxide, and wherein the process furthercomprises mixing the first and second gases in respective amounts andunder conditions sufficient substantially to neutralize the hydrogensulfide and sulfur dioxide in the following reaction:

    2H.sub.2 S+SO.sub.2 →3S+2H.sub.2 O


4. A process for producing fuel gases as claimed in claim 3 wherein themixture of first and second gases is passed to a turbine or steamgenerator to produce electricity or pressure steam.
 5. A process forproducing fuel gases as claimed in claim 4 wherein the mixture of firstand second gases is generated at a temperature between about 600°-800°C. with a pressure of between 20-30 atmospheres, the process comprisingfeeding the mixture of first and second gases to a turbine for use inproducing electricity.
 6. A process for producing fuel gases as claimedin claim 5 wherein the mixture of first and second gases is washed withwater and is then fed in a combined circle arrangement to produce energyexceeding 50% of a thermal content of the mixture of gases.
 7. A processfor producing fuel gases as claimed in claim 2 wherein the solid fuel ispyrolyzed at a temperature of between about 450°-600° C. sufficiently torelease energy in an amount about 250-600 Kcal/Kg of the lignite orpeat.
 8. A process for producing fuel gases as claimed in claim 7wherein the carbon residue of step (a) is passed for gasification instep (b) at a temperature of between about 600°-900° C. in dry form andat sufficiently high purity and surface activity to enable gasificationof the carbon residue with formation of temperatures of about900°-1,100° C.
 9. A process for producing fuel gases as claimed in claim7 wherein the carbon residue of step (a) is burned to produce pressuresteam.
 10. A process for producing fuel gases as claimed in claim 8wherein the solid fuel is pyrolyzed at a pyrolysis temperature betweenabout 400°-600° C. and thermal energy for attaining said pyrolysistemperature is provided at least in part by heat exchange with gasesproduced in step (b) and by energy released during pyrolysis.
 11. Aprocess for producing fuel gases as claimed in claim 8 wherein the solidfuel is in pulverized form and is heated prior to step (a) by thermalenergy exchange to a temperature of between about 180°-320° C. to effecta dewatering of the solid fuel by about 60-90%.
 12. A process forproducing fuel gases as claimed in claim 10 wherein the gasification instep (b) consumes only about 6% of carbon in the carbon residue and theprocess has a total thermal output of about 55 to 75% based on a thermalvalue of the solid fuel.
 13. A process for producing fuel gases asclaimed in claim 2 wherein the solid fuel contains moisture in an amountof up to 60% and the pyrolysis in step (a) effects a dewatering of thesolid fuel.
 14. A process for producing fuel gases as claimed in claim 2wherein the first gas comprises:

    ______________________________________                                        Methane          20-35%                                                       Carbon monoxide  30-50%                                                       Carbon dioxide   2-6%                                                         Hydrogen         16-22%                                                       Hydrogen Sulfide 1-3%                                                         ______________________________________                                    


15. A process for producing fuel gases as claimed in claim 14 whereinthe carbon residue is gasified in an oxidative reaction to form a secondgas in step (b), the second gas comprising:

    ______________________________________                                        Carbon monoxide  35-40%                                                       Carbon dioxide   16-22%                                                       Hydrogen         40-60%                                                       Sulfur dioxide   1-2%                                                         ______________________________________                                    


16. A process for producing fuel gases as claimed in claim 15, whereinthe first and second gases are mixed to form a gas mixture at atemperature between about 600°-800° C. and a pressure of between about20-30 atmospheres the process comprising neutralizing hydrogen sulfideand sulfur dioxide in the respective first and second gases in thefollowing reaction:

    2H.sub.2 S+SO.sub.2 →3S+H.sub.2 O


17. A process for producing fuel gases as claimed in claim 15 whereinsaid gas mixture contains thermomechanical energy that is used to drivea turbine.
 18. A process for producing fuel gases as claimed in claim 17wherein said gas mixture is also used as a fuel in a combined circlearrangement to produce electricity equivalent to about 55-75% of athermal value of the solid fuel.